Process of and apparatus for firing finegrained little reactive fuels



p 1956 G. KARDAUN ET AL PROCESS OF AND APPARATUS FOR FIRING FINEGRAINED LITTLE REACTIVE FUELS Filed Feb.

a, a MQ vG a United States Patent PROCESS OF AND APPARATUS FOR FIRING FINEGRAINED LITTLE REACTIV E FUELS Georg Kardaun, Beck, and Willem van Loon, Geleen, Netherlands, assignors to Stamicarbon N. V., Heerlen, Netherlands Application February 2, 1953, Serial No. 334,646

Claims priority, application Netherlands February 19, 1952 7 Claims. (Cl. 110-23) The present invention relates to a process and apparatus for firing fine-grained (pulverised) fuels, particularly but not exclusively weakly reactive fuels such as coke, coke dross or pulverised coal containing a low percentage of volatile matter.

Field of invention It is known that fuels containing in the region of 10% or more of volatile matter can be fired in a parallel flow system by which we mean a process in which the fuel is carried along inside the combustion chamber of a furnace in parallel flow with the combustion supporting medium which is practically always air. In such process the gas liberated from the fuel is burnt first and this primary combustion subsequently effects ignition of the carbon proper.

Hitherto however it has been found difficult to employ the parallel flow system for firing fine-grained fuels containing a low percentage of volatile matter because of the difiiculty of establishing a stable flame. The combustion of the fuels with the higher content of volatile matter can be promoted by preheating the combustion supporting air, but this. measure does not suflice in the case, e. g., of fuels containing less than 10% of volatile matter. Combustion in such cases might be promoted by heating the combustion air to a considerably higher temperature and by maintaining a low rate of feed to the furnace, but it is found that according as the feed rate is raised the fire is extinguished.

Moreover the practice of preheating the combustion air to a considerable degree is attended with technical and economic difiiculties.

For these reasons the method of firing fine grained fuels by the parallel flow system has been confined to those fuels containing suflicient volatile matter to ensure ignition and combustion of the carbon.

it has now been found possible to employ the parallel flow system for firing fine-grained fuels containing a low percentage of volatile matter in such a way that the difficulties noted above are avoided.

Objects Accordingly, the principal object of the present invention is the provision of a new process which will enable weakly reactive fuels, e. g., coke and pulven'sed coal containing a low percentage of volatile matter, to be satisfactorily fired by the parallel flow system.

A further object of the invention is the provision of a new apparatus suitable for firing fine-grained fuel, particularly weakly reactive fuel in parallel flow with the combustion supporting medium.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within 2,742,0(ll Patented Apr. 17, 1956 the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Description Briefly stated, the objects outlined above are realized according to the present invention by a process and apparatus for conducting such a process whereby the finegrained fuel particles before being fed into the furnace chamber for firing according to the parallel flow system are partially burned by feeding them continuously to a fuel bed in which the fuel particles are maintained in an incandescent and fluidised state at a temperature of from 600 to 1000 C. by means of a current of combustion air, and fuel particles are continuously withdrawn from the fluidised bed at a temperature Within the above range and fed into the furnace proper so that the combustion of the fuel is continued in the said furnace.

Due to the preheating of the fuel to be burned in the furnace which results from the combustion of part of the original fuel particles in the fluidised bed, the process may be applied to fuel containing but a low percentage, e. g., less than 10%, of volatile matter, without the fire in the furnace chamber being extinglished by the current of combustion supporting medium feeding to such chamber. If desired the fuel leaving the fluidised bed may be blown into the furnace with the aid of a current of combustion supporting medium. As an alternative the fuel may be fed, e. g., under gravity into the furnace in such a way that on its entry into the furnace chamber it is caught up by a stream of the combustion supporting medium.

The temperature to which the fuel must be preheated depends upon the character of the fuel. Pulverised coke must be preheated to a higher temperature than is required for weakly reactive pulverised coals still containing a small percentage of volatile matter.

As compared with the previously proposed process according to which the combustion air is to be preheated, the process according to the invention in which the weakly reactive fuel is preheated to the high temperature of from 600 to 1000 C. has the advantage that far less heat is required for preheating the fuel. For, to this end, for heating the fuel, the heat required is only one eighth of the heat that would be required for preheating to the high temperature of from 600 to 1000 C. the amount of combustion air which is needed for the complete combustion of the fuel.

Moreover, the combustion air is normally preheated by indirect heat exchange whereas according to the invention preheating of the fuel is effected in a simple and efficient direct manner by continuously introducing the cold fuel into a hot bed of incandescent fuel particles which, with the help of combustion air, is kept in a fluidised state, whereby a relatively small part of the total amount of fuel, say /5 by weight is burnt, while the remainder of the fuel which owing to the combustion in the fluidised bed has been preheated to the correct temperature is subsequently passed into the furnace proper for completion of combustion. The hot combustion gases, which during the partial combustion of the fuel particles are formed in the fluidised bed may be passed into the furnace proper so as there to yield their heat but these combustion gases may be utilized in some other way, e. g., they may be used for preheating boiler water or for superheating steam.

The annexed diagram serves to elucidate the process and apparatus according to the present invention.

In this figure, 1 is the furnace of a boiler, 2, constructed for effecting combustion in parallel flow. From a bunker 3 the fuel is fed into a fluidization reactor 4 with the aid of a screw conveyor. By means of an air current supplied through the pipe 5 the fuel particles are continuously kept in a fluidized state during which process the fuel particles are partially burned and the remaining fuel is introduced into the furnace 1 through the downcomer 6 and the pipe 7. Combustion air is injected into the furnace through the pipe 7. Combustion gases, from the fluid bed are led into the furnace through the pipe S; the pipes and 7 are provided with valves 9 and 10 for regulating the amount of air to be supplied and the pressure.

The ash formed is removed from the furnace through the discharge opening 11.

With an apparatus as described above pulverised. coke (grain size 0-200,u, heat of combustion 6000 k621i. kg.) was fired.

The temperature in the fluid bed reacto to which 1.5 m. of air were supplied per 1 kg. of coke, was maintained at 800 C.; by weight of the total q of coke was burned in the fluid bed reactor the rem ing coke being injected into the furnace together with fresh cold combustion air.

The process according to the invention may be carried out with any suitable kind of furnace installation for firing according to the parallel flow system. A cyclone furnace may be mentioned by way of example as being very well suited for use in association according to the invention, with a fluid bed reactor.

S ummnry The present invention provides a new and highly advantageous process and apparatus for firing fine-grained little reactive fuels.

As will be appreciated, the process of the invention offers the advantage that little reactive fuels may be combusted by maintaining a high rate of feed to the furnace, without the disadvantage that the fire will be extinguished by the flow of combustion air. Moreover by firing little reactive fuels according to the invention it is not at all necessary to preheat the combustion air as has been the practice hitherto. v

Having described our invention, what we claim is:

1. A process for burning fine-grained fuel containing a low percentage of volatile matter which comprises the steps of continuously feeding said fuel to a bed, maintaining said fuel bed in incandescent and fluidized state at a temperature of from 606 to 10%" C. by means of a current of combustion air, said fuel being partially burned in said fluidized bed, continuously withdrawing solid fuel from said fluidized bed at a temperature within the said range and completing the combustion of said solid fuel in a combustion chamber by projecting said fuel thereinto while in the incandescent state along with a current of combustion air flowing in parallel flow with said fuel.

2. A process as defined in claim 1, wherein the solid fuel withdrawn from said fluidized bed is fed to said combustion chamber by the last-mentioned current of combustion air.

3. A process as defined in claim 1, wherein the lastmentioned current of combustion air is at ambient temperature.

4. A process as defined in claim 1, wherein the hot combustion garcons products from said fluidized bed are led into said combustion chamber.

5. Apparatus for burning fine-grained fuel containing a low percentage of volatile matter comprising a reactor, means for continuously supplying fuel to said reactor, means for directing a current of combustion air upwardly through said reactor to maintain said fuel in incandescent and fluidized state therein and partially burn said fuel, a discharge pipe leading downwardly tom said reactor to withdraw solid fuel therefrom, a parallel flow furnace, means for feeding the solid fuel Withdrawn 'rorn said reactor while in the incandescent state to said furnace, and means for supplying a current of combustion air to said furnace.

6. Apparatus as defined in claim 5, including a supply pipe for said last-mentioned current of combustion air leading into said furnace, said discharge pipe coinmunicating with said supply pipe whereby said last-mentioned current of combustion air feeds the heated fuel Withdrawn from said reactor to furnace.

7. Apparatus as defined in claim 5, including closed communication means between the top of said reactor and said furnace to convey hot combustion gaseous products from said reactor to said furnace.

References \Cited in the file of this patent UNITED STATES PATENTS 943,182 Crossley et al Dec. 14, 1909 997,529 Wilson July ll, 1911 2,432,135 Barr Dec. 9, 1947 2,494,070 Thomas Jan. 1, 1950 2,506,317 Rex May 2, 1950 2,537,153 Nelson et al. Lian. 9, 1951 2,554,263 Nelson May 22, 1951 2,582,712 Howard Jan, 15, 1952 FOREIGN PATENTS 331,968 Italy Nov. 20, 1935 720,818 Germany May 16, 1942 

1. A PROCESS FOR BURNING FINE-GRAINED FUEL CONTAINING A LOW PERCENTAGE OF VOLATILE MATTER WHICH COMPRISES THE STEPS OF CONTINUOUSLY FEEDING SAID FUEL TO A BED, MAINTAINING SAID FUEL BED IN INCANDESCENT AND FLUIDIZED STATE AT A TEMPERATUR OF FROM 600* TO 1000* C. BY MEANS OF A CURRENT OF COMBUSTION AIR, SAID FUEL BEING PARTIALLY BURNED IN SAID FLUIDIZED BED, CONTINUOUSLY WITHDRAWING SOLID FUEL FROM SAID FLUIDIZED BED AT A TEMPERATURE WITHIN THE SAID RANGE AND COMPLETING THE COMBUSTION OF SAID SOLID FUEL IN A COMBUSTION CHAMBER BY PROJECTING ALONG WITH A CURRENT OF COMBUSTION AIR FLOWING IN PARALLEL FLOW WITH SAID FUEL. 